Copio is a simple, yet very useful plugin that lets you copy and paste layers in Photoshop using keyboard shortcuts. It’s as easy as selecting the layer you want to copy and hitting Ctrl + Shift + C, then go in the file you want to copy the layer to and pasting it with Ctrl + Shift + V. You can define your own keyboard shortcut for either Copy or Paste.

The plugin is free to download and use and is available both for Windows and OS X.